The present invention relates to the field of mass storage devices. More particularly, this invention relates to an apparatus and method for passing over, or more specifically slipping, defective sectors on disc surfaces within a disc drive.
One key component of any computer system is a device to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are an information storage disc that is rotated, an actuator that moves a transducer to various locations over the disc, and electrical circuitry that is used to write and read data to and from the disc. The disc drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disc.
The transducer is typically placed on a small ceramic block, also referred to as a slider, that is aerodynamically designed so that it flies over the disc. The slider is passed over the disc in a transducing relationship with the disc. Most sliders have an air-bearing surface (ABS) which includes rails and a cavity between the rails. When the disc rotates, air is dragged between the rails and the disc surface causing pressure, which forces the head away from the disc. At the same time, the air rushing past the cavity or depression in the air bearing surface produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring which produces a force on the slider directed toward the disc surface. The various forces equilibrate so the slider flies over the surface of the disc at a particular desired fly height. The fly height is the distance between the disc surface and the transducing head, which is typically the thickness of the air lubrication film. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Information representative of data is stored on the surface of the storage disc. Disc drive systems read and write information stored on tracks on storage discs. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the storage disc, read and write information on the storage discs when the transducers are accurately positioned over one of the designated tracks on the surface of the storage disc. The transducer is also said to be moved to a target track. As the storage disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing information representative of data onto the storage disc. Similarly, reading data on a storage disc is accomplished by positioning the read/write head above a target track and reading the stored material on the storage disc. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track.
Each track on a disc surface in a disc drive is further divided into a number of short arcs called sectors. The sector typically holds 512 bytes of information representing data. The number of sectors on a track used to be fixed wherever the track was located on the disc surface. Now, the number of sectors held on a track varies depending on the zone which the track is in. Typically, more sectors will be stored on the tracks at the outer diameter which are in the zone toward the outer diameter of the disc.
When a disc is manufactured, there is a possibility that there may be defects on the disc. The defects typically can result in sectors or tracks that have doubtful, dangerous, or damaged magnetic media, which would otherwise put the customer""s data at risk. These defects are to be avoided so that information representative of data is not written to a location where the data could be lost. Typically, each disc surface is checked for defects at the time of manufacture. A sector is considered defective if a number of retries must be used to recover the data on the sector. A sector is also considered defective if data written to the sector is not recoverable. The location of any defective sectors found are kept in memory on a table or map which is unavailable to end users. This table or map is used to reallocate sectors or remap the sectors on the disc drive so that, at a future time the defective sectors are not used again.
Not all defective sectors are found at the time of manufacture. Defective sectors can occur after a disc drive is manufactured and there is generally a system in place for handling newly defective sectors. Defects found after the time of manufacture are sometimes termed as grown defects. There are many sources for grown defective sectors. For example, sometimes a defective sector can be found after an inadvertent contact between the slider and the disc. Again, the location of the defective sector or track is stored in memory so that the defective sector is not reused.
There are two basic schemes or systems for handling defective sectors. One scheme is to provide spare sectors either on the track, within the cylinder or at the inner diameter of the disc. In the spare sector scheme, spare sectors could be provided on a track or within a cylinder. A cylinder is a group of tracks from different disc surfaces located at substantially the same radial distance from the center of the disc. If a defective sector was located, the location of the defect was stored in memory and the spare sector on the track or within the cylinder was located and the information representing the data was written to the spare sector. In other instances, the spare sector was not kept on the track or in the cylinder but was kept elsewhere on the surface or surfaces of the disc drive. In this scheme, a pool of spare sectors was kept off track or outside of the cylinder.
In each of these schemes, if a spare is used, the access time of the disc drive suffered. A constant industry goal is to reduce the access time to data on the disc drive. When a spare sector is used, and the spare is located on the track, the disc generally has to undergo an additional revolution to get to the spare sector, read the spare sector and then return to the point after the defective sector. If the spare is within the cylinder, many times an extra revolution is still required. If the spares on a track or within a cylinder are not used, then the disc drive wastes storage space which reduces the potential usable capacity. Placing spare sectors in a pool off track or outside of the cylinder balances wasting capacity with reduced access time. Over time, the total number of defective sectors within a disc drive are dropping. In such a drive, defective sectors are few. However, in the event of encountering a defective sector, the access time is longer than the access times associated with storing the spare within the cylinder or on the track since several seeks must be done to get to the spare sector and then back to the track from which the transducer head is reading or writing. For example, when a defective sector is found, an access must be done to the spare that replaces it, the information on the spare must be read and then a seek back to the track must be performed so that the information after the spare can be read and added to the information to complete the information or data request sent by a host computer. During each seek, a revolution may be lost which adds to the access time. If the spare sectors are kept at a location very far from the tracks being read, the actuator movements associated with the various seeks will increase the access time to the data dramatically. In addition, if there are several defective sectors in string of sectors storing a long record of information, the procedure for seeking to spares must be repeated generally as many times as there are defective sectors. For example, if there are three discontinuous defective sectors, the process of accessing to the spare, reading it and accessing back to the track where the defective sector was found must be done three times to complete a request for information or data. Access times suffer greatly in this situation.
Another scheme is called defect slipping. This scheme of handling defects is generally used at the time of manufacture of the disc drive. In essence, when a defective sector is found, the defective sector is skipped and the next good sector available is used as the next sector. Defect slipping skips over defective sectors and keeps the locations of the defects in a large table. The good sectors used are sequentially numbered as logical block addresses. Large buffer memories are used to store a large table for mapping each logical block address to a physical location on the disc surface. Compared to the spare sector method, this procedure reduces the number of seeks necessary to obtain the information. One of the problems associated with this method is that it is generally unavailable for xe2x80x9cgrown defectsxe2x80x9d which occur after manufacture. Since the defective sectors are merely skipped and the next sector is slipped to a position past the defective sector, the defect slipping technique requires that the sectors are available to slip to. This situation occurs at manufacture. After manufacture, most of the sectors are usually designated as good sectors and few spare sectors are positioned within the disc drive so that they are available to be slipped to. If spares were provided to allow for defect slipping after manufacture, large amounts of potential storage capacity would be wasted. Consequently, after manufacture, defect slipping is generally not a viable option for handling defective sectors.
What is needed is a method and apparatus for dealing with defective sectors that reduces access times. What is also needed is a process which reduces the number of seeks needed to recover information from a pool of spare sectors for long records of data or for information that include multiple defective sectors. Furthermore, there is a need for a scheme for handling defective sectors that keeps wasted storage space to a minimum, yet accurately substitutes spare sectors in for defective sectors within the disc drive.
A method for writing a plurality of sectors of information to a storage surface on a disc in a disc drive includes determining the start location for writing the plurality of sectors; and determining a number of skipped defective sectors that will be encountered when writing the plurality of sectors. A first defective sector is skipped and the next logical block address is written to the next non defective sector after the first defective sector. A second defective sector is skipped and the next logical block address is written to the next non defective sector after the second defective sector. At least two sectors of the plurality of sectors are reassigned and written to at least two sectors in a spare sector pool. At least two of the last sectors of the plurality of sectors to be written are reassigned to two sectors within the spare sector pool. The first defective sector and the second defective sector are noncontiguous sectors. The sectors of the spare sector pool may be contiguous sectors within the spare sector pool. The sectors in the spare sector pool also may be located a track on the disc. The method includes storing the location of the spare sectors within the spare sector pool so that the spare sectors may be located at a subsequent time during a read operation.
At least the number of skipped sectors of the plurality of sectors to be written are written to a spare sector pool located a track on the disc. A number of the sectors at the end of the plurality of sectors equal to the number of skipped sectors of the plurality of sectors are reassigned and written to a number of sectors in the spare sector pool equal to the number of skipped sectors of the plurality of sectors. The number of sectors in the spare sector pool is also equal to the number of skipped sectors of the plurality of sectors that were to be written.
A defect management system for a disc drive for managing defective sectors located on the disc surface of a disc drive includes a controller, and a memory associated with said controller. The memory also includes a first portion storing locations of a plurality of defective sectors, and a second portion for storing an instruction set which controls a head during a write operation for writing a plurality of sectors. The controller skips at least a first defective sector and a second defective sector. At least one writable sector is located between the first defective sector and the second defective sector. The controller also controls the head to write at least the last two sectors of the plurality of sectors to be written to at least two contiguous sectors located in a pool of spare sectors.
Advantageously, the disc drive which uses the above inventions reduces the access times associated with obtaining information from sectors that include defective sectors which need to be reassigned. The actuator movements associated with reading from a set of sectors having reassigned sectors many times will be reduced to one extra seek to a set of spare sectors. This reduction in access time can significantly enhance the performance of the disc drive, especially if there are a large number of reassigned sectors on the disc drive. The time for writing information representative of data using this method requires no additional time and may require less time. The method and apparatus also efficiently and accurately accesses the spare sectors from the spare sector pool located on the disc drive.